dart2js: Use frequency of occurence to sort metadata indices.

pkg/compiler/lib/src/js_emitter/helpers.dart

Index: pkg/compiler/lib/src/js_emitter/helpers.dart
diff --git a/pkg/compiler/lib/src/js_emitter/helpers.dart b/pkg/compiler/lib/src/js_emitter/helpers.dart
index 017094b71868868de7518f2ac556f5451fdbb024..a5591ace9b0bc766fc17068d788a55eb956d025a 100644
--- a/pkg/compiler/lib/src/js_emitter/helpers.dart
+++ b/pkg/compiler/lib/src/js_emitter/helpers.dart
@@ -12,3 +12,22 @@ ClassElement computeMixinClass(MixinApplicationElement mixinApplication) {
   }
   return mixin;
 }
+
+class _DeferredOutputUnitHash extends jsAst.DeferredString {
+  String _hash;
+  final OutputUnit _outputUnit;
+
+  _DeferredOutputUnitHash(this._outputUnit);
+
+  void setHash(String hash) {
+    assert(_hash == null);
+    _hash = hash;
+  }
+
+  String get value {
+    assert(_hash != null);
+    return '"$_hash"';
+  }
+
+  String toString() => "HashCode for ${_outputUnit} [$_hash]";
+}